[Illustration: OBSIDIAN CLIFF, Jim Bridger’s famous “mountain of glass.” The rock is rhyolite lava which contains a high proportion of obsidian, a kind of black volcanic glass. Note columnar jointing along the sides of the cliff, similar to that shown by the basalt flows at Tower (fig. 33). The cliff is approximately 200 feet high. (Fig. 29)] [Illustration: THICK RHYOLITE LAVA FLOW along west bank of Firehole River. (Fig. 30)] [Illustration: Closeup view is of a cut surface of rhyolite, showing the striking banding that results from the flowage of viscous molten rock. The dark bands are chiefly concentrations of volcanic glass (also some cavities), and the light bands are concentrations of tiny crystals of feldspar and quartz.] [Illustration: BRECCIATED RHYOLITE LAVA FLOWS along the Firehole Canyon drive. As a lava flow moves outward from its center of eruption, a chilled crust develops along its upper surface and outer edges because of the cooler temperatures in those parts of the flow. Continued movement of the still-molten rock in the interior of the flow causes this crust to break up (brecciate) into angular blocks. The blocks are then tumbled along until the whole mass finally solidifies. (Fig. 31)] [Illustration: OUTCROP OF GLASSY RHYOLITE LAVA along the road between Canyon Village and Norris Junction. The conspicuous lines in the face of the rock outline different layers produced by lava flowage. The feldspar crystals are alined parallel to the direction of flow. (Fig. 32)] [Illustration: In closeup A, dark parts of the rock are volcanic glass (closeup B shows glassy fracture) and light-colored crystals are quartz (blocky) and feldspar (tabular).] [Illustration: Closeup B.] About 30 different flows have been recognized. Grouped within a major rock unit called the Plateau Rhyolite (fig. 5), they cover more than 1,000 square miles. The gently rolling plateau surface of central Yellowstone, broken here and there by clusters of low-lying hills and ridges, is essentially the landscape that characterized the upper surfaces of the lava flows soon after they cooled and solidified. Natural valleys formed between some of the adjacent flows, and in places streams still follow these readymade channels. Rhyolite, in both lava flows and ash-flow tuffs, is by far the predominant rock type seen along the Park roads.